Recent advances in medical technology have often involved the measurement or recording of electrical activity which occurs as a function of underlying biological activity. This in turn has led to considerable research and development in the field of sensing electrodes adapted for use in a number of applications where biopotentials are measured on the skin or tissue of a living organism. Silver-silver chloride electrodes combined with an electroconductive paste or gel have been found to be particularly suitable for biopotential measurements, as illustrated by U.S. Pat. Nos. 4,328,809 to B. H. Hirschowitz, et al. and 4,270,543 to K. Tabuchi et al. Electrodes of this type have been previously assembled in matrix configurations for use in screening and mapping applications as illustrated by U.S. Pat. Nos. 4,416,288 to Freeman, 4,486,835 to Bai and 4,628,937 to Hess et al.
Many electrodes are packaged in a pre-gelled state wherein an electrolytic paste or gel is packaged as part of the electrode. The gel may be located in a central gel reservoir consisting of a molded cup, or it may be contained in a dye-cut hole in a foam which encapsulates a gel saturated open cell compressible foam column, such as shown by U.S. Pat. No. 3,868,946. In most instances, the pre-gelled electrodes are sold ready for use with an electrically conductive material such as metal or a metal chloride in contact with the electrolyte gel.
A pre-gelled electrode system is generally not a battery by itself, but forms a part of a battery-system consisting of two or more electrodes placed on the body. In such a system, a complex battery is formed consisting of many interactive components including the electrode material (frequently silver/silver chloride), the electrode gel, internal body chemistry and external skin conditions, skin preparation, temperature, air condition and chemistry, etc. Obviously, some of these factors are not subject to control, but in order to get the best data possible, especially in instances where DC biopotentials are of interest, artifacts, such as DC offsets, should be reduced to the lowest level. Clearly, pre-gelled electrodes can possibly represent such undesired DC voltage artifact which should be limited to the lowest voltage possible; ideally zero volts. Most pre-gelled electrodes when introduced in the battery system outlined above contribute some unwanted DC voltage (polarization effect) to biopotential measurements. It is important to lower the possibility of such DC artifacts occurring in a degree sufficient to have a substantial adverse effect on biopotential measurements.
It is not feasible, in situ, to measure only the DC contributed by the pre-gelled electrodes when they are placed in a battery system. Instead, manufacturers and researchers have attempted to establish the magnitude of these potentials by mounting two pre-gelled electrodes under test in a back to back fashion with the electrolyte gels of the two electrodes touching and the leads from the electrodes connected to a voltmeter. In fact, this is a fair measurement since the technique does create a battery by using two electrodes and eliminating the body. When pre-gelled electrodes are tested this way, using a high impedance digital voltmeter, DC voltages to approximately 10 millivolts are frequently found.
Many attempts have been made to reduce the polarization effect present in biopotential electrodes. This was first done by modifying the electrode structure in an attempt to avoid dissimilar junction effects as illustrated by U.S. Pat. No. 3,496,929 to F. J. Domingues. Subsequently, an oxidizing agent has been added to the electrolyte gel to reduce the metal on the surface of the electrode sensing element to a metal action which reacts with the anion of the electrolyte to produce an insoluble compound which is deposited on the sensing element to render it non-polarizable. This method of producing a non-polarizable electrode is illustrated by U.S. Pat. No. 4,377,170 to H. M. Carim.
Although the prior art structures for reducing the polarization effect present with biopotential electrodes accomplish this purpose to some extent, they require a basic chemical material change in the electrodes, and do not effectively eliminate offset potentials which can cause a significant error in a biopotential measurement taken with the electrode. Prior art structures do not provide a simple, removable method for depolarizing various types of pre-gelled electrodes. Also, these prior structures do not provide either an efficient or cost effective method for depolarizing a large number of electrodes combined in an electrode mapping or screening matrix.